Proposal: Allow setting ulimits for containers

Add option to set ulimit settings for containers.
Currently the container inherits from the daemon's ulimit settings.

Support for this was added in libcontainer here docker/libcontainer@7ce34f58cbd4884634a0f1f82c4aaab1168ea0de
Sorry for the super nitpicky remark, but I don't think the trailing comma is legal JSON.